The role of excitatory amino acids, such as glutamic acid and aspartic acid, as the predominant mediators of excitatory synaptic transmission in the central nervous system has been well established. Watkins and Evans, Ann. Rev. Pharmacol. Toxicol., 21, 165 (1981); Monaghan, Bridges, and Cotman, Ann. Rev. Pharmacol. Toxicol., 29, 365 (1989); Watkins, Krogsgaard-Larsen, and Honore, Trans. Pharm. Sci., 11, 25 (1990). These amino acids function in synaptic transmission primarily through excitatory amino acid receptors. The excitatory amino acids also participate in a variety of other physiological processes such as motor control, respiration, cardiovascular regulation, sensory perception, and cognition.
Excitatory amino acid receptors are classified into two general types. Receptors that are directly coupled to the opening of cation channels in the cell membrane of the neurons are termed "ionotropic." This type of receptor has been subdivided into at least three subtypes, which are defined by the depolarizing actions of the selective antagonists N-methyl-D-aspartate (NMDA), .alpha.-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA). The second general type of receptor is the G-protein or second messenger-linked "metabotropic" excitatory amino acid receptor. This second type, when activated by the agonists quisqualate, ibotenate, or trans-1-aminocyclopentane-1,3-dicarboxylic acid, leads to enhanced phosphoinositide hydrolysis in the postsynaptic cell. Both types of receptors appear not only to mediate normal synaptic transmission along excitatory pathways, but also participate in the modification of synaptic connections during development and changes in the efficiency of synaptic transmission throughout life. Schoepp, Bockaert, and Sladeczek, Trends in Pharmacol. Sci., 11, 508 (1990); McDonald and Johnson, Brain Research Reviews, 15, 41 (1990).
The excessive or inappropriate stimulation of excitatory amino acid receptors leads to neuronal cell damage or loss by way of a mechanism known as excitotoxicity. This process has been suggested to mediate neuronal degeneration in a variety of conditions. The medical consequences of such neuronal degeneration makes the abatement of these degenerative neurological processes an important therapeutic goal.
Excitatory amino acid excitotoxicity has been implicated in the pathophysiology of a number of neurological disorders. This excitotoxicity has been implicated in the pathophysiology of acute and chronic neurodegenerative conditions including cerebral deficits subsequent to cardiac bypass surgery and grafting, stroke, cerebral ischemia, spinal cord trauma, head trauma, Alzheimer's Disease, Huntington's Chorea, amyotrophic lateral sclerosis, AIDS-induced dementia, perinatal hypoxia, cardiac arrest, hypoglycemic neuronal damage, ocular damage and retinopathy, and idiopathic and drug-induced Parkinson's Disease. Other neurological conditions, that are caused by glutamate dysfunction, require neuromodulation. These other neurological conditions include muscular spasms, migraine headaches, urinary incontinence, psychosis, opiate tolerance and withdrawal, anxiety, emesis, brain edema, chronic pain, convulsions, and tardive dyskinesia. The use of a neuroprotective agent, such as an AMPA or NMDA receptor antagonist, is believed to be useful in treating these disorders and/or reducing the amount of neurological damage associated with these disorders. The excitatory amino acid antagonists are also useful as analgesic agents.
Recent studies have shown that AMPA receptor antagonists are neuroprotective in focal and global ischemia models. The competitive AMPA receptor antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline) has been reported effective in preventing global and focal ischemic damage. Sheardown et al., Science, 247, 571 (1900); Buchan et al., Neuroreport, 2, 473 (1991); LePeillet et al., Brain Research, 571, 115 (1992). The noncompetitive AMPA receptor antagonist GKYI 52466 has been shown to be an effective neuroprotective agent in rat global ischemia models. LaPeillet et al., Brain Research, 571, 115 (1992).
Recent studies have shown that NMDA receptor antagonists are neuroprotective in animal models of focal cerebral ischemia. Bullock and Fujisawa, Journal of Neurotrauma, 9 (supplement 2), S443 (1992); Scatton et al., Cerebrovascular Disease, 1, 121 (1991). These studies have shown that the competitive NMDA antagonist D-(-)CPP-ene provided protection in a focal cerebral ischemia model in cats, the competitive NMDA antagonist CGS 19755 provided protection in a focal cerebral ischemia model in rats, and the competitive NMDA antagonist LY233053 provided protection in a CNS ischemia model in rabbits. Bullock et al., Journal of Cerebral Blood Flow and Metabolism, 10, 668 (1990); Simon and Shirasho, Annals of Neurology, 27, 606 (1990); Madden et al., Journal of Neurosurgery, 76, 106 (1992). The non-competitive NMDA antagonist dizocilpine provided protection in models of focal cerebral ischemia in cats and rats. Park et al., Journal of Cerebral Blood Flow and Metabolism, 8, 757 (1988); Park et al., Annals of Neurology, 24, 543 ( 1988). The competitive NMDA antagonist LY274614 is neuroprotective in an animal model of Huntington's Disease. Schoepp, et al., Journal of Neural Transmission [General Section], 85, 131 (1991).
Several studies have shown that NMDA antagonists are anticonvulsant agents. Meldrum, Epilepsy Research, 12, 189 (1992); Meldrum, Epilepsia, 32 (supplement 2), S1 (1991); Chapman and Maidrum, New Antiepileptic Drugs (Epilepsy Research Supplement 3), Elsevier, 39 (1991). For example, the competitive NMDA antagonists D-(-)CPP-ene and CGP 37849 are anticonvulsant against sound induced seizures in DBA/2 mice. Chapman, Graham, and Meldrum, European Journal of Pharmacology, 178, 97 (1990). Other studies have shown that NMDA antagonists are analgesics. For example, the competitive NMDA antagonist CGS 19755 is analgesic in a warm water tail withdrawal procedure in rhesus monkeys and the competitive NMDA antagonist DL-AP5 was analgesic in a mouse formalin model. France, Winger, and Woods, Brain Research, 526, 355 (1990); Murray, Cowan, and Larson, Pain, 44, 179 (1991).
These studies strongly suggest that the delayed neuronal degeneration in brain ischemia involves glutamate excitotoxicity mediated at least in part by AMPA and/or NMDA receptor activation. Thus, AMPA and NMDA receptor antagonists may prove useful as neuroprotective agents and improve the neurological outcome of cerebral ischemia in humans.